A non-aqueous electrolyte secondary battery typified by a lithium ion secondary battery has a high energy density. Thus, a non-aqueous electrolyte secondary battery is currently being widely used as a battery for use in devices such as a personal computer, a mobile phone, and a mobile information terminal.
However, a non-aqueous electrolyte secondary battery may be heated by a flow of a high current in a case where an internal short circuit or an external short circuit is caused by breakage of the non-aqueous electrolyte secondary battery or breakage of a device in which the non-aqueous electrolyte secondary battery is used. Thus, a non-aqueous electrolyte secondary battery is required to ensure a high level of safety by preventing an internal short circuit due to, for example, breakage of the non-aqueous electrolyte secondary battery while maintaining various performance capabilities such as a rate characteristic and a resistance characteristic (solution resistance) of the non-aqueous electrolyte secondary battery.
Examples of a proposed non-aqueous electrolyte secondary battery whose safety has been improved by preventing an internal short circuit include: (1) a non-aqueous electrolyte secondary battery in which an inorganic compound containing flaky particles is dispersed in a separator and the inorganic compound is oriented so as to be substantially parallel to a surface of the separator (Patent Literature 1); (2) a non-aqueous electrolyte secondary battery in which an inorganic particle layer containing spherical inorganic particles and amorphous inorganic particles is laminated to an electrode surface (Patent Literature 2); and (3) a non-aqueous electrolyte secondary battery in which a porous layer containing: a first insulating inorganic filler containing a metallic hydroxide and/or a hydrate of a metallic oxide; and a second insulating inorganic filler having a thermal conductivity of not less than 10 W/m·K is laminated to a porous film (Patent Literature 3).
Note that examples of a proposed method for evaluating safety of a non-aqueous electrolyte secondary battery include: (1) a safety evaluation method by placing a foreign object at a location inside a non-aqueous electrolyte secondary battery at which location a cathode and an anode face each other, and pressing the location to locally crush an insulating layer interposed between the cathode and the anode, thereby causing an internal short circuit (Patent Literature 4); (2) a safety evaluation method by inserting, from a pointed head of a non-aqueous electrolyte secondary battery into a depth where an internal short circuit is generated, an insulated bar including an electrically conductive member (Patent Literature 5); and (3) a safety evaluation method by impressing a load on a needle equipped with a tip part made of a conductive member, inserting the tip part of the needle into an insulation layer laminated on an electrode, making the tip part to reach the electrode, and measuring, at time lapses, the load as well as an electric resistance between the tip part of the needle and the electrode (Patent Literature 6).